Optical communication involves the transmission of information by sending pulses of light, typically through an optical fiber cable. A laser diode is commonly used to transmit data in digital form over a telecommunications network. The light forms a carrier wave that is modulated to carry information. Optical transmission involves generating an optical signal independent of changes in the laser diode's operating environmental conditions or aging factors over time. Optical communications are known for low-loss and high data-carrying capacity. However, the quality of optical signal must be controlled in order to achieve the advantages of the optical communication system.
A laser driver circuit is used to control the transmission of light from the laser diode. One implementation of a closed-loop optical modulation amplitude (OMA) controller involves a laser monitor photodiode (MPD), as described in U.S. Pat. No. 9,300,405 ('405 patent). OMA is the difference between two optical power levels of a digital signal generated by an optical source, e.g., the laser diode. The '405 patent describes an analog implementation of circuitry which correlates the AC components of the monitoring signal with the data signal. The OMA controller can operate together with an average power controller to provide a complete laser driver system.
The block diagram of the OMA controller described in the '405 patent is shown in FIG. 1. A comparison is made between a low-frequency component of the monitoring signal IMPD and a low-frequency component of the data signal IREF to generate a residue signal IRESIDUE The residue signal is converted to a voltage using a transimpedance amplifier (TIA) and compared to the data signal in the OMA controller which ultimately controls the bias and modulation currents of the laser driver to drive the residue signal to zero. The residue signal and the data signal are filtered to remove high frequency components of the signal and AC coupled to remove the DC component of the signal in filters 50 and 54. The filtered signals are compared by multiplying (mixing) or correlating these two signals to generate a modulation control feedback signal.
Previous implementations of the system have used analog circuitry to implement the filtering, mixing and modulation control shown in blocks 50, 54, 56, 60, and 62. The output of integrator 60 is typically sampled at fixed, prescribed intervals, governed by a clock generator. At each clock period, the sign of the integrator output is used to decide whether to increment or decrement the Mod DAC value, and then integrator 60 is reset. The chosen clock period is a compromise between the required response speed and initialization time of the OMA control loop, and accuracy or noise rejection.
The analog implementation is limited in terms of accuracy, particularly at greater data rates. The analog signal is subject to DC offset, mismatches, and other errors, which reduces resolution in the output signal. Monitor photo diode 70 cannot resolve individual logic one and logic zero periods, particularly at high data rates.